Introduction

Impedance microbiology is a microbiological technique used to measure the microbial number density (mainly bacteria but also yeasts) of a sample by monitoring the electrical parameters of the growth medium. The ability of microbial metabolism to change the electrical conductivity of the growth medium was discovered by Stewart and further studied by other scientists such as Oker-Blom, Parson and Allison in the first half of 20th century. However, it was only in the late 1970s that, thanks to computer-controlled systems used to monitor impedance, the technique showed its full potential, as discussed in the works of Fistenberg-Eden & Eden, Ur & Brown and Cady.

Eighty strains representing four starter lactic acid bacteria species, Lactobacillus helveticus, Lactobacillus delbrueckii subsp. bulgaricus, Lactococcus lactis, and Streptococcus thermophilus, were analyzed by impedance measurements. The strains, belonging to the collection of the Laboratory of Food Microbiology of the Department of Food Science of University of Parma, have been previously isolated from dairy matrixes and identified by16S rRNA sequencing.

The responses of microorganisms to specific environmental conditions, such as temperature, pH and aw, can be described by predictive microbiology, a sub-discipline of food microbiology dealing with the development of mathematical models. Several models have been developed to represent and predict microbial growth or inactivation in food and, nowadays, such models can be very useful in food technology and processing since they are applied to predict the outcome of fermentation processes under particular circumstances and to assess the effects of environmental conditions on microbial growth.

Examples of primary models, widely applied to describe the growth of lactic acid bacteria, include sigmoidal equations, such as Logistic and Modified Gompertz models. This describes the changes of the microbial population density as a function of time using a limited number of kinetic parameters (e.g., lag time, growth or inactivation rate and maximum population density) while it is not taken into account the stage of death. The Gompertz model provides a convenient mathematical tool that approximates the way in which microbiologists have traditionally estimated the graph of the growth kinetics.

Impedancimetric method

The suitability of impedance measurements for detecting and enumerating certain microorganisms in food has been tested many times, the main emphasis being on the detection or enumeration of Enterobacteriaceae and the determination of the total bacterial count. A smaller number of publications are dedicated to the detection or enumeration of other bacterial groups or bacterial species that are important in the food industry, such as Listeria, clostridia, or lactobacilli.

The impedancimetric method, using the interface capacitance curves, is applicable for the detection and quantification of raw cow's milk bacterial content. With it, we can obtain greater growth variations, threshold detection times somewhat shorter, and a coefficient of correlation between IC and TDT slightly but significantly better than those obtained by total conductance curves, thus offering some advantage.

It has been found that the greater the number of different strains from various species used in a test, the poorer is the correlation between impedance enumeration and conventional colony counts. This is a result of the differing metabolic activities of the various species involved.

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Medical Microbiology & Diagnosis

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